1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an X-ray source-for emitting an X-ray beam in order to form an X-ray image of an object, an X-ray detector for deriving an optical image from the X-ray image, an image pick-up apparatus including one or more image sensors for deriving electronic sub-image signals from sub-images of the optical image, and a combination unit for combining the electronic sub-image signals so as to form an electronic image signal for a composite image. The invention also relates to an image pick-up apparatus including one or more image sensors for deriving electronic sub-image signals from sub-images and a combination unit for combining the electronic sub-image signals so as to form an electronic image signal for a composite image. The invention also relates to a method of composing a composite image from a first sub-image picked up by a first image sensor and a second sub-image picked up by a second image sensor.
2. Description of the Related Art
An image pick-up apparatus and a method of this kind are known from German Offenlegungsschrft DE 33 15 882.
The known X-ray examination apparatus including an X-ray source and an X-ray image intensifier whereto the image pick-up apparatus is optically coupled. An X-ray image is formed of an object, for example a patient to be radiologically examined who is arranged between the X-ray source and the X-ray image intensifier, by irradiating the object by means of an X-ray beam emitted by the X-ray source. The X-ray image is formed on an entrance screen of the X-ray image intensifier and converted into an optical image on an exit window of the X-ray image intensifier. The optical image is picked up by the image pick-up apparatus and converted into an electronic image signal. The light emanating from the exit screen is split into two sub-images by means of an image splitter including a partly transparent mirror, said sub-images being picked up by respective image sensors.
The known image pick-up apparatus includes two solid-state image sensors, each of which has a plurality of photosensitive elements. The image sensors are arranged in such a manner that pixels of the optical image which are picked up by one image sensor will be situated in the intermediate spaces between the photosensitive elements of the other sensor. Each image sensor picks up a respective sub-image and applies a respective sub-image signal to the combination unit which forms an image signal for a composite image from said sub-image signals. From the sub-images the image sensors derive electronic sub-image signals having signal levels which represent brightness values in the sub-images. The composite image consists of image lines of the sub-images which have been picked up by the individual sensors, i.e. in such a manner that image lines from one image sensor constitute the odd image lines in the composite image and image lines from the other sensor constitute the even image lines in the composite image. In the direction transversely of the image lines the composite image has a spatial resolution which has approximately been doubled in comparison with the sub-images picked up by the individual sensors.
The known image pick-up apparatus has the drawback that, even when the optical image has a uniform brightness, image lines from different image sensors may have different brightness values in the composite image because differences occur between signal levels of sub-image signals, even though they relate to brightness values from substantially the same positions on the exit window. The differences may be due inter alia to the fact that the individual image sensors receive light from the exit window via different optical paths with differences in light attenuation. Another cause of differences between brightness values of image lines from different image sensors is a difference between sensitivities of the individual image sensors. Such differences cause disturbances in the composite image. The composite image may exhibit, for example a streaky pattern which is not at all related to the image information in the optical image but is caused by unequal intensities of the sub-images or by unequal sensitivities of the individual image sensors. Due to the above causes, individual image sensors derive image signals having different signal levels from a light intensity in substantially the same position on the exit window. Two differences contribute to the difference between said signal levels: an offset difference and a gain difference.